1. Field of the Invention
The present invention relates to a sheet feeding apparatus provided in an image forming apparatus such as a copying machine or a laser beam printer for forming an image on a sheet, and relates to an image forming apparatus including the sheet feeding apparatus.
2. Description of the Related Art
Conventionally, in a sheet feeding apparatus provided in an image forming apparatus, in general, an uppermost sheet of sheets stacked on a rising and lowering plate is sequentially fed to an image forming portion by a feed roller. In such a sheet feeding apparatus, the rising and lowering plate, which is provided so as to be swingable, is urged by a coil spring in a direction of the feed roller to bring an uppermost surface of a stack of the sheets stacked on the rising and lowering plate into press-contact with the feed roller.
Further, U.S. Pat. No. 5,253,854 proposes the following sheet feeding apparatus. Specifically, the sheet feeding apparatus includes the feed cams fixed coaxially with the feed roller. While the feed roller rotates to send out a sheet, the feed cams push down the rising and lowering plate to a certain position against an urging force of the coil spring.
As described above, in the sheet feeding apparatus including the mechanism for pushing down the rising and lowering plate by the feed cams, at the time of waiting, the rising and lowering plate can be kept pushed down at the certain position. Thus, setting and replacement of the sheets are facilitated, and by pushing down the rising and lowering plate while sending out the sheet, a separating property of the sheet is improved at the separating portion such as the separating pad.
With reference to FIG. 17, operations of the sheet feeding apparatus performed in the image forming apparatus from sheet feeding to sheet delivery after image formation will be briefly described.
As illustrated in FIG. 17, sheets S set on a feed tray 1 are each sent out in such a manner that a feed roller 2 is rotated through transmission of drive of a drive motor 16, and one of the sheets is separated by a separating pad. The fed sheet S is conveyed via conveyance rollers 3a and 3b to a transfer nip as an image transferring portion formed by a photosensitive drum 8 and a transfer roller 9. A rising and lowering plate 22 provided in the feed tray 1 can rise and lower, and is urged upward by a feed spring 23. Cams 21 are provided coaxially with the feed roller 2, and are held in slide-contact with cam followers 22b provided on the rising and lowering plate 22. Further, when the feed roller 2 rotates, the cams 21 rotate to raise and lower the rising and lowering plate 22 through the cam followers 22b, and the rising and lowering plate 22 rises to bring the sheet S into press-contact with the feed roller 2, to thereby send out the sheet S.
On the other hand, an image writing laser scanner 5 forms an electrostatic latent image on the photosensitive drum 8 in a process cartridge 7 to prepare a toner image. Onto the sheet S, the toner image formed on the photosensitive drum 8 is transferred as an unfixed image at the transfer nip formed by the photosensitive drum 8 and the transfer roller 9. In order to heat and fix the unfixed image, the sheet S is conveyed to fixing rollers 11, and the unfixed image is heated and fixed onto the sheet S. The sheet S subjected to image fixing is conveyed toward a delivery roller 12 along a conveyance guide 15 so as to be delivered onto a delivery tray 14.
However, in the conventional example described above, in a case of increasing rotation speed of the feed roller 2 for increasing feeding speed of the sheet, or in a case of reducing a diameter of the feed roller 2 for downsizing the sheet feeding apparatus, speed of swing movement of the rising and lowering plate 22 is increased in association with this increase or reduction. As a result, there is a fear in that noise is increased when the sheets abut on the feed roller 2, or a fear in that through repetition of feeding operations, an alignment property of the stacked sheets is deteriorated due to impact caused when the sheets abut on the feed roller 2. In a case where the alignment property of the sheets is deteriorated, there is a fear in that skew occurs at the time of sheet feeding, with the result that image failure, jamming, and the like are likely to occur.